Process for making encapsulant for opto-electronic devices

ABSTRACT

An encapsulant for use with opto-electronic devices and optical components incorporates a filler made from a glass that has been processed into particle form and heated to a predetermined temperature for a predetermined time, along with an epoxy having an index of refraction matched to that of the glass and heated to a predetermined temperature for a predetermined time, to prevent settling of the filler particles after mixing the filler particles with the epoxy, and thereby obtaining uniform dispersion of the particles within the epoxy. The encapsulant provides for high light transmittance, and its coefficient of thermal expansion can be varied by varying the amount of filler without substantially altering the optical properties of the encapsulant. The coefficient of thermal expansion variation within the encapsulant preferably is less than 30%, due to uniform dispersion of the filler particles within the epoxy.

This is a division of U.S. patent application Ser. No. 10/455,733, filedJun. 4, 2003 now U.S. Pat. No. 6,841,888.

BACKGROUND OF THE INVENTION

The present invention relates generally to encapsulants foropto-electronic devices, and more particularly, to such encapsulantsexhibiting superior optical qualities. The present invention alsorelates to methods for making such encapsulants.

Optical and electronic (i.e., “opto-electronic”) devices, such as LEDs,photodetectors, and fiber optic components, generally are encapsulatedusing a variety of materials to protect the devices from vibration,humidity, heat, environmental deterioration, electrical leakages, andother deteriorative factors. A suitable encapsulant should possess anumber of particular characteristics, described in detail below.

The encapsulant should have a coefficient of thermal expansion (CTE)preferably lower than 50 ppm/° C. Also, variation in the CTE should belower than ±30% over the entire volume of the encapsulant. If the CTEand/or its variation are greater than these values, the encapsulant canexcessively expand or contract when exposed to varying temperatures,thereby causing breakage of the device or its leads.

The encapsulant also should have the highest possible lighttransmittance, preferably at wavelengths between 300 nm and 800 nm. Inparticular, the encapsulant preferably should exhibit lighttransmittance higher than 65% at about 650 nm for an encapsulant havingthickness of about 1 mm. In contrast, commercially availableencapsulants often have substantially lower transmittance, and thereforeare translucent, or even opaque. This can lead to difficulty when theseencapsulants are used with opto-electronic devices requiring lighttransmission.

Additionally, the encapsulant should have a glass transition temperature(T_(g)) preferably higher than 120° C. Because opto-electronic devicesare subjected to temperatures substantially higher than usual ambienttemperatures, the encapsulant can flow when its glass transitiontemperature is exceeded, resulting in destruction of the encapsulateddevice.

The encapsulant also should have high electrical resistivity, to providesufficient insulation for the opto-electronic devices. It should furtherprovide adequate electrical insulation to protect the opto-electronicdevices from the effects of adverse environmental conditions, such asheat and humidity. Finally, the encapsulant should be inexpensive andeasy to produce using readily available materials.

Transparent epoxy resin compositions having refractive indices (i.e.n_(D) at 25° C.) varying between 1.48 and 1.60 at a wavelength of about588 nm can be prepared by curing commercially available chemicalcompounds incorporating epoxy groups. These inexpensive commercial epoxyresins also are known to exhibit high light transmittance, high T_(g),high electrical resistivity, and high heat resistance. However, theseepoxy resins usually have CTE higher than 50 ppm/° C., and thereforethey are not suitable for use as encapsulants for opto-electronicdevices.

Encapsulants having low CTE can be prepared by mixing commerciallyavailable uncured epoxy compounds with inorganic fillers, followed bycuring of this mixture. For example, U.S. Pat. No. 3,547,871 to Hofmanet al. describes a low CTE encapsulant comprising an epoxy resin and afiller having a particle size ranging between 10 μm and 300 μm. Thisencapsulant resin has a viscosity below 20,000 cP at 100° C. The filleris selected from silica, fused quartz, beryllium aluminum silicate,lithium aluminum silicate, or mixtures of these. The claimedencapsulants have a CTE lower than 50 ppm/° C. However, the Hofman '871patent does not disclose an encapsulant having high light transmittance.

An encapsulant having both low CTE and high light transmittance can beprepared by incorporating a filler and epoxy having the characteristicsdescribed as follows. The filler should be uniformly dispersed in theepoxy to provide a uniform filler CTE over the entire volume of theencapsulant, to prevent eventual damage of the opto-electronic devicefrom the cumulative effect of operation at varying temperatures. Toobtain uniform particle dispersion and produce an encapsulant having auniform CTE, the filler should have a predetermined and consistentparticle size, and the liquid epoxy should have a predeterminedviscosity. If the filler particle size is smaller than about 1 μm, theparticles tend to agglomerate, resulting in non-uniform particledispersions and entrapment of gas bubbles in the encapsulant. Thisresults in lowered light transmittance of the encapsulant. Suchagglomeration and bubble formation might be avoided by reducing theamount of the filler used in the encapsulant. However, reducing filleramount tends to increase the CTE of the encapsulant above 50 ppm/° C.Therefore, preferably agglomeration is avoided by using fillers havingparticle sizes greater than about 1 μm. Unfortunately, if fillerparticle size is larger than about 1 μm, the particles can settle at thebottom of the encapsulant during preparation and casting of theencapsulant due to gravity. This settling will occur if epoxy viscosityis too low during preparation of the encapsulant layer, leading tonon-uniform distribution and non-uniform CTE in the encapsulant.Conversely, if epoxy viscosity is too high, forming the encapsulantlayer over the opto-electronic device can be difficult. Therefore, theviscosity of the liquid epoxy should be within a specified range inorder for the material to be useful in preparation of the encapsulant.

In addition to uniform dispersion, the refractive index of the fillershould closely match the refractive index of the epoxy resin at thecured stage to have high light transmittance. Inorganic fillersfrequently are used to lower costs or enhance mechanical properties ofthe resins. However, their use also can lead to decreased opticaltransmittance and scattering of light by the medium produced. Thisscattering can be decreased, and the optical transmittance of the mediumcan be increased, by closely matching the refractive index of atransparent filler used with that of a transparent resin. Because therefractive index of these filler particles generally cannot be directlymeasured with sufficient precision, this matching generally must beperformed by trial and error, adjusting the filler or epoxy as needed.

Also, the filler should be free of chemical compounds that can reducethe electrical resistivity of the encapsulant below an acceptable levelunder the temperature and humidity conditions of the device. Some ofthese compounds have inherently low electrical resistivity, while otherscan decompose into or form electrically conductive ions. Heat andhumidity can affect this inherent electrical resistivity or thisdecomposition, as well as migration of the ions. Therefore, preparationof the filler from such compounds should be avoided. Finally, the fillershould be manufactured easily and inexpensively.

Single-component particles of inorganic metal oxides, such as SiO₂,TiO₂, and ZrO₂, are known to be easily and inexpensively prepared usingvapor deposition or solution precipitation processes. These particlescan be used as fillers in encapsulants. However, these particles areoften less than 1 μm in size, and therefore generally are unsuitable foruse as fillers for the reasons described above. Furthermore, mostsingle-component particles cannot be used as fillers, because they havefixed refractive indices. For example, SiO₂ particles have a fixedrefractive index of 1.42, TiO₂ (rutile form) of 2.3, and ZrO₂ of 1.95.These indices cannot be adjusted to match the refractive index of the,epoxy resin used for making the encapsulant.

It is possible to adjust the refractive index of fillers made frommulti-component glass particles. For example, U.S. Pat. No. 5,175,199 toAsano et al. describes a sol-gel method for making a multi-componentglass filler to be mixed into a transparent epoxy, which can be used asan encapsulant for optical semiconductor devices. In this method,TiO₂—SiO₂ gel is synthesized by hydrolyzing and condensing a siliconalkoxide and a titanium alkoxide. This gel is dried, and then eitherground into particulate matter, followed by sintering to dense glassbeads, or sintered into a dense glass followed by grinding into glassbeads. This sintering is achieved at a very high temperature range of1,050° C. to 1,250° C. The disclosed filler manufacturing method has thedisadvantages of being complicated and expensive, as well as requiringlengthy preparation time and high sintering temperature. Therefore,production of these fillers is expensive. Additional disadvantagesinclude possible phase separation, crystallization, and coloring of TiO₂at the high sintering temperatures required using these methods. Phaseseparation and crystallization cause intolerably high refractive indexdifferences between the glass filler and the epoxy, thereby lowering thetransmittance. TiO₂ is also known to possibly cause yellowing of organicresins in which it is included as a result of extended exposure tolight. This leads to degradation of the transmittance of the encapsulantover time.

U.S. Pat. No. 5,198,479 to Shiobara et al. uses the method described inthe Asano '199 patent described above, and it further discloses a methodto overcome the discoloration problems of the TiO₂—SiO₂ fillersdiscussed above, by addition of organic phosphorus anti-discoloringagents into the uncured epoxy-filler composition. This addition, whileeffective, further complicates use of the method described in the Asano'199 patent, and the resulting filler therefore is more expensive.

European Patent No. 0 391 447 B1 to Nakahara et al. teaches a sol-gelmethod for the production of multi-component metal oxide particles thatcan be used as fillers in transparent organic resins. The Nakahara etal. patent incorporates the step of first preparing seed particles ofsingle-component metal oxides, then growing these particles by additionof hydrolyzable and condensable organic metal compounds such as metalalkoxides, to prepare multi-component particles such as TiO₂—SiO₂,ZrO₂—SiO₂, and Al₂O₃—SiO₂. This process is complicated and expensive,and the fillers produced are expensive. Furthermore, because theseparticles are smaller than 1 μm in size, it is difficult to obtain bothhomogeneous bubble-free dispersions of these particles in the epoxy andlow CTE.

U.S. Pat. No. 5,618,872 to Pohl et al. discloses a method for makingmulti-component encapsulant filler particles for opto-electronicdevices, comprising two or more oxides selected from SiO₂, TiO₂, ZrO₂,Al₂O₃, V₂O₅, and Nb₂O₅. The method for particle preparation described inthis patent is similar to that described in the Nakahara et al. patent,and it shares the same drawbacks. Therefore, it is not suitable forpreparation of high transmittance encapsulants.

Dunlap and Howe, in Polymer Composites, vol. 12(1), pp. 39-47, (1991),describe a casting composition comprising a resin and an index-matchedfiller prepared by ball milling of a glass. The size of the fillerparticles ranges between 2 μm and 100 μm. Subsequent to ball milling,the filler particles are annealed at temperatures between 0° C. and 10°C. above the glass strain point for at least one hour to removestresses, as well as organic contaminants. The inventors have found thatheat treatments at temperatures above the strain point of the glass canreduce transmittance, and therefore such temperatures should be avoided.Furthermore, Dunlap and Howe do not disclose an encapsulant having auniform CTE and a method for preparing such an encapsulant.

Japanese Patent Publication No. 11-074424 to Yutaka et al. discusses amethod for making an encapsulant for use in a photo semiconductordevice. In this method, a silica powder containing PbO or TiO₂ having aparticle size ranging between 3 μm and 60 μm is used as an index-matchedfiller for an epoxy resin composition. PbO or TiO₂ can causecrystallization during manufacturing of these multi-component glassfillers, thereby decreasing the transmittance of the filled epoxy. Inaddition to the aforementioned disadvantages of using TiO₂ as a fillermaterial, PbO is known to be a health hazard, and its use inmanufacturing of the encapsulants should therefore be avoided.

U.S. Pat. No. 6,246,123 to Landers et al. describes an encapsulanthaving high transmittance, low CTE, and low T_(g). The encapsulant ismade from a polymer resin and an index-matched filler. However, thefiller is selected from a group consisting of alkali zinc borosilicateglasses. The presence of alkali ions is know to potentially reduceelectrical resistivity of encapsulants, leading to high leakage currentsand possible damage to the encapsulated device. For example, U.S. Pat.No. 4,358,552 to Shinohara et al. explains that encapsulantsincorporating low levels of alkali contaminants, such as Li⁺, Na⁺, K⁺,and ionic contaminants, such as Cl, improves electrical insulation ofthe encapsulated electronic device.

Naganuma et al. in Journal of Material Science Letters, vol. 18, pp.1587-1589, (1999) describes preparation of encapsulants foropto-electronic devices by mixing an epoxy with a filler prepared from amulticomponent glass, SiO₂—Al₂O₃—B₂O₃—MgO—CaO having an average particlesize of 26 μm or 85 μm. During the described curing of the epoxy and thefiller mixture, the mold is turned over every 10 minutes to preventsegregation of the filler. However, these encapsulants do not havetransmittance higher than 65% at a CTE lower than 50 ppm/° C., andtherefore are not suitable for use in the manufacture of opto-electronicdevices.

It should be appreciated from the foregoing description that thereremains a need for a transparent encapsulant having uniform and low CTEover its entire volume comprising inexpensive index-matched fillers andepoxy resins. The fillers should be able to be uniformly dispersed toprovide an average CTE lower than 50 ppm/° C., with a CTE variation ofless than ±30% over the entire volume of the encapsulant to preventthermal expansion damage. The encapsulant also should have atransmittance higher than 65% to minimize signal losses, and T_(g)higher than 120° C. to reduce physical stress and breakage, therebypreventing the opto-electronic device from damage. Finally, theencapsulant should exhibit high electrical resistivity under varyingtemperature and humidity conditions. The present invention fulfills thisneed and provides further advantages.

SUMMARY OF THE INVENTION

The present invention resides in an encapsulant for an opto-electronicdevice or optical component, the encapsulant characterized by acoefficient of thermal expansion and an optical transmittance. Theencapsulant incorporates a filler consisting essentially of glassparticles having diameters in the range of 1 μm to 500 μm formed from aglass essentially free of titania. The glass is characterized by a glassrefractive index having a value in the range of 1.48 to 1.60 and avariance of less than about 0.001. The filler is characterized by afiller refractive index. The encapsulant also incorporates an epoxycharacterized by an epoxy refractive index. The coefficient of thermalexpansion of the encapsulant has an average value of less than 50 ppm/°C., and more preferably less than 40 ppm/° C. The filler and epoxyrefractive indices have values sufficiently similar such that theoptical transmittance of the encapsulant is at least 65% when measuredat a wavelength in the range of 300 nm to 800 nm at an encapsulantthickness of about 1 mm.

In preferred embodiments of the present invention, the coefficient ofthermal expansion of the encapsulant preferably has a variation of lessthan ±30%, and more preferably less than ±10%. The optical transmittanceof the encapsulant preferably is at least 75%, more preferably 80%, whenmeasured at a wavelength in the range of 300 nm to 800 nm at anencapsulant thickness of about 1 mm. The optical transmittance of theencapsulant is at least 65%, more preferably 75%, and most preferably80%, when measured at a wavelength in the range of 600 nm to 700 nm atan encapsulant thickness of about 1 mm. The optical transmittance of theencapsulant preferably is at least 65%, more preferably 75%, and mostpreferably 80%, when measured at a wavelength of about 650 nm at anencapsulant thickness of about 1 mm.

The glass particles in the encapsulant preferably are essentially freeof alkali, most preferably consisting essentially of borosilicate. Anembodiment of the present invention is an opto-electronic devicesubstantially encased in such an encapsulant. Preferably, the glassrefractive index has a value of about 1.526. The glass particles havediameters in the range of 1 μm to 250 μm, and more preferably 10 μm to250 μm. Preferably, less than 60 volume percent of the fillerincorporates glass particles having diameters smaller than 10 μm, morepreferably less than 20 volume percent, most preferably less than 10volume percent.

In preferred embodiments of the encapsulant, the filler comprisesbetween 5 volume percent and 60 volume percent of the total volume ofencapsulant, more preferably 10 volume percent and 50 volume percent,and most preferably between 15 volume percent and 40 volume percent.Preferably, the filler has been heat treated at a heat treatmenttemperature for at least about 1 hour, more preferably between 5 hoursand 50 hours, and most preferably between 30 hours and 40 hours. Theheat treatment temperature is at about the strain point of the glass orless, and more preferably between 20° C. and the strain point of theglass. Preferred encapsulants have been heat treated at a heat treatmenttemperature of about 627° C. or less, more preferably between 20° C. and627° C., and most preferably between 450° C. and 550° C. This heattreatment preferably takes place in an oxygen-containing atmosphere,preferably dry air.

In preferred embodiments of the invention, the filler is reacted with asilane coupling agent selected from the group consisting ofaminopropyl-triethoxysilane, vinyltrimethoxysilane, methacryloxypropyltriethoxysilane, gylcidoxypropyltrimethoxysilane, and mixtures ofthese. Most preferred is amino-propyltriethoxysilane.

The epoxy is prepared from a composition incorporating: a) diglycidylether of bisphenol-A resin; b) cyclo-aliphatic epoxy resin; c) ahardener; and d) an accelerator. The ratio by weight of diglycidyl etherof bisphenol-A resin to combined weight of diglycidyl ether ofbisphenol-A resin and cyclo-aliphatic epoxy resin preferably has a valuein the range of 0.20 to 0.54, and more preferably 0.30 to 0.50. Theratio by weight of hardener to combined weight of diglycidyl ether ofbisphenol-A resin and cyclo-aliphatic epoxy resin has a value in therange of 0.30 to 0.80, and more preferably 0.50 to 0.70. The ratio byweight of accelerator to combined weight of diglycidyl ether ofbisphenol-A resin and cyclo-aliphatic epoxy resin preferably has a valuein the range of 0.001 and 0.05, and more preferably 0.005 and 0.03.

A preferred embodiment of the present invention is an encapsulant for anopto-electronic device or optical component, the encapsulantcharacterized by a coefficient of thermal expansion and an opticaltransmittance, the encapsulant incorporating: between 15 volume percentand 40 volume percent of a filler and 2) between 60 volume percent and85 volume percent of an epoxy. The filler consists essentially of glassparticles formed from an alkali-free borosilicate glass that has beensilylated with amino-propyltriethoxysilane, with the particles havingdiameters in the range of 1 μm to 250 μm. Less than 10 percent by volumeof the particles have diameters less than 10 μm, and the glass ischaracterized by a glass refractive index having a value of about 1.526and a variance of less than about 0.001. The filler has been heated inan oxygen-containing atmosphere at a temperature in the range of 450° C.to 550° C. for a duration in the range of 30 hours to 40 hours. Theepoxy is prepared from a composition incorporating diglycidyl ether ofbisphenol-A resin, cyclo-aliphatic epoxy resin, a hardener, and anaccelerator. The ratio by weight of diglycidyl ether of bisphenol-Aresin to combined weight of diglycidyl ether of bisphenol-A resin andcyclo-aliphatic epoxy resin has a value in the range of 0.30 to 0.50,the-ratio by weight of hardener to combined weight of diglycidyl etherof bisphenol-A resin and cyclo-aliphatic epoxy resin has a value in therange of 0.50 to 0.70, and the ratio by weight of accelerator tocombined weight of diglycidyl ether of bisphenol-A resin andcyclo-aliphatic epoxy resin has a value in the range of 0.005 and 0.03.The coefficient of thermal expansion of the encapsulant has an averagevalue of less than about 40 ppm/° C. and a variation of less than about±10%, and the optical transmittance of the encapsulant is at least 80%when measured at a wavelength of 650 nm at an encapsulant thickness of 1mm.

The present invention also resides in method for an encapsulant for anopto-electronic device or optical component incorporating the followingsteps: 1) processing a glass characterized by a glass refractive indexin the range of 1.48 to 1.60 with a variance of less than about 0.001 toform filler particles from the glass having diameters between 1 μm and500 μm and characterized by a filler refractive index; 2) heating thefiller particles in an oxygen-containing atmosphere; 3) preparing anepoxy characterized by an epoxy refractive index, so that the filler andepoxy refractive indices are sufficiently similar such the encapsulanthas optical transmittance of at least 65% when measured at a wavelengthin the range of 300 nm to 800 nm at an encapsulant thickness of about 1mm; 4) heating the epoxy to a predetermined temperature for apredetermined duration sufficient to increase the viscosity of the epoxyto a level such that the epoxy is characterized by a settling velocityequal to or greater than a predetermined value; 5) mixing the epoxy withthe filler particles within a predetermined mixing duration to form theencapsulant; 6) cooling the encapsulant to a predetermined temperaturesufficient to increase the viscosity of the epoxy in the encapsulantwithin a predetermined cooling duration; and 7) removing air bubblesfrom the encapsulant within a predetermined defoaming duration.

Preferred aspects of the method incorporate processing the glass to formfiller particles having diameters between 1 μm and 250 μm, such thatless than 60 percent by volume, more preferably 20 percent, and mostpreferably 10 percent, of the filler particles have diameters less than10 μm. Preferably, the method incorporates processing the glass to formfiller particles having diameters between 10 μm and 250 μm. The methodpreferably incorporates heating the filler particles comprises heatingthe filler particles to the temperatures and for the durations discussedabove, preferably in dry air. The method also preferably incorporatesreacting the glass particles with a silane coupling agent after the stepof processing the glass, such as amninopropyltriethoxysilane.

The step of preparing an epoxy preferably incorporates mixing together adiglycidyl ether of bisphenol-A resin, a cyclo-aliphatic resin, ahardener, and an accelerator. Preferably to prepare an epoxy to have asufficient viscosity, such that the settling velocity of the fillerparticles in the encapsulant is less than about 11 mm/min, morepreferably 6 mm/min, most preferably 4 mm/min. The method can furtherincorporate heating the epoxy to a temperature between 80° C. and 140°C., more preferably between 90° C. and 110° C., most preferably about100° C. for a predetermined duration sufficient to increase theviscosity of the epoxy before the step of mixing the epoxy with thefiller particles.

In preferred aspects of the invention, the step of heating the epoxyincorporates heating the epoxy for a duration sufficient to increase theviscosity of the epoxy to be in the range of 300 cP to 40,000 cP, morepreferably 500 cP to 20,000 cP, and most preferably 750 cP to 10,000 cP.The step of cooling the encapsulant preferably incorporates cooling theencapsulant to a temperature sufficient to increases the viscosity ofthe epoxy to a value in the range of 1,000 cP to 40,000 cP, morepreferably 5,000 cP to 20,000 cP, and most preferably 7,000 cP to 12,000cP. Preferably, the steps of mixing, cooling, and removing air bubbleshave a combined duration of less than about 120 minutes, more preferably60 minutes, and most preferably 30 minutes.

A particularly preferred method for making an encapsulant for anopto-electronic device or optical component within the scope of thepresent invention incorporates the following steps: 1) processing aborosilicate glass characterized by a refractive index having a value ofabout 1.526 when measured at a wavelength of about 588 nm and a varianceof less than about 0.001, to form borosilicate glass particles havingdiameters between 1 μm and 250 μm, such that less than 60 percent byvolume of the borosilicate glass particles have diameters less than 10μm; 2) heating the borosilicate glass particles to a temperature between450° C. and 550° C. for a duration between 30 hours and 40 hours in anoxygen-containing atmosphere; 3) reacting the borosilicate glassparticles with aminopropyltriethoxysilane; 4) preparing an epoxy bymixing diglycidyl ether of bisphenol-A resin, cyclo-aliphatic resin,hexahydrophthalic anhydride curing agent, and triphenylphosphitecatalyst; 5) heating the epoxy to about 100° C. for a durationsufficient to increase the viscosity of the epoxy to between 750 cP and10,000 cP to obtain a settling velocity of the borosilicate glassparticles in the epoxy to less than 4 mm/min; 6) mixing the epoxy andthe borosilicate glass particles; 7) cooling the encapsulant to atemperature sufficient to further increase the viscosity of the liquidepoxy to a level between 7,000 cP and 12,000 cP; and 8) removing airbubbles from the encapsulant. The steps, of mixing, cooling, andremoving in this preferred method have a combined duration of less thanabout 30 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the relation between thereaction time, the reaction temperature, and the viscosity of threeliquid epoxies after addition of catalyst. The epoxies were prepared inthe same manner as described in EXAMPLE 1, except that they were reactedat about 90° C. for about 255 minutes, at about 100° C. for about 182minutes, and at about 110° C. for about 103.5 minutes, respectively.

FIG. 2 is a graphical representation of the relation between the coolingtemperature and the viscosity of a liquid epoxy prepared in the samemanner as described in EXAMPLE 1, reacted at 100° C. for about 120minutes after addition of the catalyst.

FIG. 3 is a schematic representation of the method for preparation ofthe borosilicate glass filler as described in EXAMPLE 1.

FIG. 4 is a graphical representation of the size distribution of fillerparticles prepared within the scope of the present invention, after thestep of ball milling.

FIG. 5 is a graphical representation of the size distribution of fillerparticles prepared within the scope of the present invention, after thesteps of sieving and solvent washing.

FIG. 6 is a schematic representation of the method for preparation ofthe epoxy as described in EXAMPLE 1.

FIG. 7 is a schematic representation of the method for preparation ofthe encapsulant as described in EXAMPLE 1.

FIG. 8 is a graphical representation of the size distribution of fillerparticles prepared within the scope of the present invention, obtainedby filtering the solvent used for washing in the step of solventwashing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention resides in a highly transparent encapsulant foruse with opto-electronic devices incorporating a glass filler and anepoxy, and an opto-electronic device substantially encased in theencapsulant. The encapsulant is suitable for use with opto-electronicdevices, such as LEDs, photodetectors, and fiber optic devices andoptical components such as lenses, prisms, and mirror substrates. Thefiller is made from commercial glass particles having predeterminedparticle size that are heat treated under predetermined conditions toimprove the optical properties of the filler. The epoxy is prepared fromconventional materials in weight ratios selected such that the resultingencapsulant is highly transparent, because the refractive index of theepoxy closely matches the refractive index of the filler. Theencapsulant of the present invention offers a unique combination of lowand uniform CTE and high transparency, along with high electricalresistivity. The present invention also resides in a method for makingthis encapsulant.

The filler is prepared by crushing and grinding a TiO₂—free commercialglass. The refractive index of this glass can range between 1.48 and1.60, as measured at a wavelength of about 588 nm and a temperature ofabout 25° C. To be suitable for use as a filler in the encapsulant ofthe present invention, this glass must have a substantially homogeneousrefractive index. Excessive variation in the refractive index of thefiller, which would result if the glass used to make the filler hadexcessive variation in its refractive index, would make it impossible toprepare a filler that closely and consistently matches the refractiveindex of the epoxy. Preferably, the refractive index variation of theglass should be less than 0.001. High electrical resistivity can beachieved using glasses substantially free of alkali ions or otherelectrically conductive ions. The preferred filler used in theencapsulant of the present invention is made from a commerciallyavailable alkali-free borosilicate glass having a typical composition byweight of 10% to 50% SiO₂, 10% to 50% BaO, 10% to 20% Al₂O₃, and 10% to20% B₂O₃. An example of a commercially-available alkali-freeborosilicate glass suitable for use in the present invention is marketedunder the trade name AF45 from Schott Corporation of Yonkers, New York.Other TiO₂—free glasses containing alkali ions, such as soda limeglasses, can be used for preparation of filler particles forencapsulants for electronic devices that can tolerate low electricalresistivity, or for optical components, such as lenses, for which norequirements for electrical insulation exist.

To prepare the filler, the alkali-free and titania-free glass first iscrushed into coarse, irregular particles approximately 5 mm or less insize. These coarse particles then are ball-milled into finer particlesusing conventional methods. These finer particles are first screened andthen solvent-washed. The particles are finally dried and taken for useas the filler material. These particles have sizes preferably between 1μm and 500 μm, more preferably between 1 μm and 250 μm, and mostpreferably between 10 μm and 250 μm. The filler particles therebyobtained then are heat-treated, at temperatures preferably lower thanthe strain point of the glass. The borosilicate fillers are heat treatedpreferably below the strain point of the borosilicate glass, about 627°C., more preferably between 20° C. and 627° C., and most preferablybetween 450° C. and 550° C. The heat treatment is carried out for aduration of at least 1 hour, more preferably between 5 hours and 50hours, and most preferably between 30 hours and 40 hours. To provide forimproved optical properties in the filler material, this heat treatmentpreferably takes place in an oxygen-containing atmosphere, mostpreferably in dry air composed of about 21% O2 and about 79% N2 with amoisture content less than 60 ppm. After the heat treatment, the filleris mixed into an epoxy prepared as described below.

Optionally, the miscibility and bonding of the filler particles with theepoxy can be increased by using a suitable silane coupling agent. As iswell known in the art, for example, in Silane Coupling Agents by EdwinP. Plueddemann (Plenum Press, New York, 1991), these agents aid inchemical bonding of an organic resin, such as epoxy, with an inorganicmaterial, such as glass. This bonding can prevent formation of voids atthe glass and resin interface by increasing miscibility and therebytransmittance. This bonding also can increase moisture resistance andmechanical strength of the encapsulant. Examples of such agents areamino-propyltriethoxysilane, vinyltrimethoxysilane, methacryloxypropyltriethoxysilane, and gylcidoxypropyltrimethoxysilane.Aminopropyltriethoxysilane is most preferred of these. These agents canbe mixed with components of the encapsulant at different stages of itspreparation. For example, these agents can be reacted with fillerparticles prior to their mixing with the epoxy, or they can be mixedwith the epoxy before mixing the epoxy with the filler. In the mostpreferred process, the filler particles are first reacted withaminopropyltriethoxysilane in a mixer, then heat treated at about 100°C. for about 1 hour, and finally dispersed in the epoxy.

The epoxy is made from a composition incorporating the following fourcomponents, generally known for manufacture of these epoxies, designatedas (a) to (d) below.

(a) Diglycidyl ether of bisphenol-A (DGEBPA) resins, such as EPON 825 orEPON 3002 marketed by Shell Chemical Company of Houston, Tex.

(b) Cyclo-aliphatic epoxy resin, such as CY 179 MA marketed by CibaSpecialty Chemicals, Tarrytown, N.Y., or ERL-4221E marketed by DowChemical Company, Midland, Mich.

(c) A hardener (curing agent), such as hexahydrophthalic anhydride(HHPA) marketed by Lonza Chemicals, Basel, Switzerland; and

(d) An accelerator (catalyst), such as triphenylphosphite (TPPP)marketed by GE Specialty Chemicals, Morgantown, W.Va., or zinc stearatemarketed by Crompton Corporation, Greenwich, Conn.

Components (a) to (d) listed above have different refractive indices.They are combined in specific proportions to provide an epoxy having arefractive index closely matching to that of the filler described above.As a practical matter, it is not possible to directly measure therefractive index of the filler particles with sufficient precision, andthis index may not be identical to that of the glass from which thefiller is prepared. Therefore, the proportions of components (a) to (d)are determined experimentally, so that the resulting encapsulantprovides for acceptable, and preferably maximum, light transmittancewhen the particular epoxy is used with a particular filler. To preparesuitable epoxies for use with the present invention, the following threeratios R (for resin), H (for hardener), and A (for accelerator) aredefined for the amounts by weight of components (a), (b), (c), and (d)to be used in the epoxy:R=a/(a+b)H=c/(a+b)A=d/(a+b)

To provide epoxies having refractive indices sufficiently similar tothat of fillers produced as described above resulting in acceptabletransmittance, preferably R ranges between 0.20 and 0.54, H rangesbetween 0.3 and 0.8, and A ranges between 0.001 and0.05. Mostpreferably, R ranges between 0.3 and 0.5, H ranges between 0.5 and 0.7,and A ranges between 0.005 and 0.03.

Mixing of the filler and the liquid epoxy under the considerationsdiscussed above produces the encapsulants of the present invention. Theliquid encapsulant can then be placed over a suitable opto-electronicdevice using various methods known in the art, such as dip-coating,potting, casting, and transfer molding. Eventual solidification andposturing of this liquid encapsulant yields an opto-electronic deviceencased in the encapsulants of the present invention.

After mixing of the filler with the liquid epoxy and before thesolidification of this suspension, the filler particles may settle downat the bottom of the liquid due to gravity. This settling causesnon-uniform distribution of the filler particles in the encapsulant,which results in, non-uniform CTE and can eventually damage theencapsulated device during use. This settling can be partially orcompletely prevented by slowing down the settling velocity of theparticles in a liquid epoxy. The settling velocity of solid particles ina liquid can be calculated from Stokes' law,

$u_{t} = \frac{D_{p}^{2}{g\left( {P_{p} - P} \right)}}{18\mu}$

-   -   where, u_(t) is the settling velocity, D_(P) is the        characteristic diameter of the filler particle, g is the        gravitational constant, P_(P) is the density of the filler        particle, P is the density of the liquid encapsulant, and μ is        the viscosity of the liquid encapsulant. Table 1 below provides        the settling velocities in units of mm/min of the borosilicate        particles in the liquid epoxy mixture described below in        EXAMPLE 1. In EXAMPLE 1, the borosilicate glass has a density of        2.72 g/cm³, and the epoxy has a density of 1.32 g/cm³. The        settling velocities in Table 1 are calculated for particle sizes        varying between 1 μm and 500 μm, and for viscosity of the liquid        epoxy varying between 100 cP and 20,000 cP.

TABLE 1 Liquid epoxy Borosilicate Filler Particle Size, μm Viscosity, cP1 20 50 100 250 500 100 0.0005 0.183 1.145 4.578 28.613 114.450 2500.0002 0.073 0.458 1.831 11.445 45.780 500 0.0001 0.037 0.229 0.9165.723 22.890 750 0.0001 0.024 0.153 0.610 3.815 15.260 1,000 0.00000.018 0.114 0.458 2.861 11.445 2,000 0.0000 0.009 0.057 0.229 1.4315.723 3,000 0.0000 0.006 0.038 0.153 0.954 3.815 4,000 0.0000 0.0050.029 0.114 0.715 2.861 5,000 0.0000 0.004 0.023 0.092 0.572 2.28910,000 0.0000 0.002 0.011 0.046 0.286 1.145 20,000 0.0000 0.001 0.0060.023 0.143 0.572

To achieve uniform dispersion of the filler particles in the solidencapsulant, the settling velocities preferably should be less than 11mm/min, more preferably less than 6 mm/min, and most preferably lessthan 4 mm/min. For example, according to Table 1, such settlingvelocities can be achieved by using borosilicate particles having sizespreferably less than 500 μm, and most preferably less than 250 μm, andliquid epoxy viscosities preferably higher than 300 cP, more preferablyhigher than 500 cP, and most preferably higher than 750 cP.

Use of very small filler particles also should be avoided to preventparticle agglomerations or bubble entrapments that can cause non-uniformparticle dispersion or low transmittance. Therefore, particle sizespreferably should be larger than 1 μm, and most preferably larger than10 μm.

Very high liquid epoxy viscosities should be avoided to easily mix thefiller particles with the liquid epoxy, and to easily defoam theresulting suspension. During the mixing, air bubbles can be entrapped inthe encapsulant. These bubbles should be removed from the liquid toincrease transmittance. This is achieved using various methods known inthe art, such as defoaming in vacuum and bubbling with helium.Therefore, the epoxy viscosities preferably should be lower than about40,000 cP, more preferably lower than about 20,000 cP, and mostpreferably lower than about 10,000 cP for easy mixing of the epoxy withthe filler and to defoam the suspension.

Thus, to obtain uniform dispersion of the filler in the encapsulant, toprevent entrapment of air bubbles, and to achieve uniform encapsulantCTEs having variations preferably below about ±30%, and most preferablybelow about ±10%, the filler particle sizes preferably should be between1 μm and 500 μm, more preferably between 1 μm and 250 μm, and mostpreferably between 10 μm and 250 μm; and the liquid epoxy viscositiespreferably should be between 300 cP and 40,000 cP, more preferablybetween 500 cP and 20,000 cP, and most preferably between 750 cP and10,000 cP.

The liquid epoxies having viscosities preferred for use in theencapsulants of the present invention are obtained by reacting theliquid epoxy after addition of the catalyst at a temperature in therange of preferably 80° C. to 140° C., more preferably 90° C. to 110°C., and most preferably about 100° C., for a predetermined period. As anexample, FIG. 1 shows the relationship between reaction time, reactiontemperature, and the liquid epoxy viscosity. In this example, thereaction times in minutes required to attain the preferred liquid epoxybefore addition of the filler particles, are shown in Table 2.

TABLE 2 Reaction Liquid Epoxy Viscocity Temperature 300 cP 500 cP 750 cP 90° C. 65 100 128 100° C. 65 85 108 110° C. 51 60 68

Before mixing, the glass filler is heated to a temperature preferablybetween 20° C. and the reaction temperature of the liquid epoxy, morepreferably between 50° C. and the reaction temperature of the epoxy, andmost preferably between 60° C. and the reaction temperature of theliquid epoxy. After mixing, the liquid suspension is cooled to apredetermined temperature, to further increase the viscosity of theliquid epoxy and thereby prevent the settling of the filler particles.As an example of this, the relationship between the cooling temperatureand the viscosity of the liquid epoxy is illustrated in FIG. 2. Thecooling temperature is predetermined according to the viscosity of theliquid epoxy. Cooling temperatures that yield viscosities preferablybetween 1,000 cP and 40,000 cP, more preferably between 5,000 cP and20,000 cP, and most preferably between 7,000 and 12,000 cP are withinthe scope of this invention. The suspension then is defoamed at thispredetermined cooling temperature. The suspension is preferably stirredduring the cooling and the defoaming to prevent the settling of thefiller particles.

The cooling and defoaming of the suspension should be completedpreferably within about 120 minutes, more preferably within about 60minutes, and most preferably within about 30 minutes, to prevent thesettling of the filler particles in the liquid epoxy.

After completion of defoaming, the liquid encapsulant is placed on thesurfaces of suitable opto-electronic devices using various methods knownin the art, such as dip-coating, potting, casting, and transfer molding.Eventual solidification of the liquid suspension and a step of posturingyield the encapsulated opto-electronic device.

By matching the refractive index of the epoxy to that of the filler, anencapsulating material with high transmittance is obtained. Thistransmittance is preferably higher than about 65%, more preferablyhigher than about 75% and most preferably higher than about 80%,preferably at a wavelength range of 300 nm and 800 nm, more preferablyat a wavelength range of 600 nm and 700 nm, and most preferably at 650nm, when measured across an about 1 mm thick sample of encapsulant. Mostexamples of preferred encapsulants made within the scope of the presentinvention exhibit transmittances higher than about 85%.

Because the filler and the epoxy are prepared to have closely matchedrefractive indexes, the relative amount of the filler material in theencapsulant can be varied substantially without having significanteffect on the light transmittance of the encapsulant. Therefore, therelative amount of filler can be varied to adjust the CTE of theresulting encapsulant to preferably less than about 50 ppm/C and mostpreferably less than about 40 ppm/C. The relative amount of filler inthe encapsulant preferably ranges between 5 volume percent and 60 volumepercent of the encapsulant, more preferably between 10 volume percentand 50 volume percent, and most preferably between 15 volume percent and40 volume percent.

In addition to the advantages described above, the most preferredencapsulants, which incorporate alkali-free borosilicate glass in thefiller, have a high electrical resistivity, in contrast to otherpossible glasses. Encapsulants produced according to the method have ahigh tolerance for humid environments (i.e., up to 85% relative humidityat a temperature of 85 C), which results in stable optical and physicalproperties under a variety of environmental conditions.

Because of their low CTE and high transmittance, encapsulants within thescope of the present invention also can be used in the manufacturing ofoptical components such as lenses, prisms, mirror substrates, andsimilar products. When not used with an electronic device or in anapplication for which electrical insulation properties are notimportant, such encapsulants also can incorporate high-alkali glasses asfillers.

EXAMPLE 1

An encapsulant within the scope of the present invention is prepared inthis illustrative Example.

Preparation of the Filler Particles:

The process of preparation of the filler particles is schematicallyillustrated in FIG. 3. A commercial alkali-free borosilicate glasssheet, marketed under the trade name AF45 by Schott Corporation, havingan average refractive index of about 1.526 at about 588 nm and at about25° C., and a refractive index variation within the glass of less thanabout 0.001, was crushed into coarse particles, and then sieved througha #8 mesh sieve to eliminate all particles larger than about 2.36 mm indiameter. These sieved particles were placed in a 100 ml zirconiamilling jar, along with 3 milling balls having diameters of 20 mm and 15milling balls having diameters of 15 mm. The particles then wereplanetary ball-milled at about 200 RPM for about 30 minutes. The sizedistribution of particles obtained after the step of ball-milling wasdetermined using a particle size analyzer (model LA-900, marketed byHoriba Incorporated of Irvine, Calif.). As shown in FIG. 4, the particlesizes of the resulting white powder ranged between 1 μm and 300 μm, witha peak at about 50 μm.

Next, all of the milled particles larger than about 250 μm in diameterwere removed via sieving. The content of particles smaller than about 10μm in diameter in the filler was decreased via a solvent wash. Thiswashed powder was dried at a temperature of about 160 C for about 12hours. The size distribution of particles obtained after the steps ofsolvent washing and drying is shown in FIG. 5. The particle sizes of theresulting white powder ranged between 2 μm and 250 μm, with a peak atabout 50 μm. Six volume percent of this powder was composed of particleshaving sizes smaller than about 10 μm. Then, the particles were heattreated at a temperature of about 500° C. for about 36 hours in dry air.

After heat treatment, the dried filler particles were silylated -with asilane coupling agent, as follows. The filler particles were first mixedwith aminopropyl-triethoxysilane in a Keyence mixer. (Model No. HM-501,marketed by Keyence Corporation of Osaka, Japan) for about 2 minutes ata room temperature of about 20° C. In this preparation, about 1 gram ofaminopropyltriethoxysilane was used per about 99 grams of filler.Finally, the particles were heat treated at a temperature of about 60 Cfor about 1 hour in air to complete the step of silylation.

Preparation of the Epoxy:

The process of preparation of the liquid epoxy is schematicallyillustrated in FIG. 6. This epoxy was prepared using the followingingredients: a) EPON 3002 resin; b) CY 1.79 MA resin; c) HHPA curingagent; and d) TPPP catalyst. The epoxy was prepared by combining about43 grams of EPON 3002 and about 64.5 grams of CY 179 MA in a closedcontainer. The container was placed in an ethylene glycol bath at atemperature of about 100 C, and the two components were mixed at about300 RPM for about 60 minutes. Next, about 67.5 grams of HHPA were addedto the solution, and the solution was stirred at about 300 RPM for about5 minutes. Finally, about 1.08 gram of the catalyst, TPPP was added tothe solution, and the solution was stirred for about 120 minutes atabout 300 RPM, to form the epoxy. With respect to the previously-definedratios, the composition of this epoxy provided for values of R of about0.40, H of about 0.63, and A of about 0.01.

In addition to the preparation described above, three additional epoxieswere prepared in the same manner. After addition of the catalyst, theseepoxies were reacted at about 90° C. for about 255 minutes, at about 100C for about 182 minutes, and at about 110° C. for 103.5 minutesrespectively. The effect of the reaction time and the reactiontemperature on the viscosity of these liquid epoxies is illustrated inFIG. 1. The viscosity of the epoxy was measured using a viscometer(Model No. LVDV-II+), a spindle (Model No. SC4-18), and a chamber (ModelNo. SC4-13RPY), all manufactured by Brookfield Engineering LabsIncorporated of Staughton, Mass. After mixing the catalyst, the liquidepoxies each were placed into the chamber of the viscometer, which hadbeen heated to the reaction temperature. A shear rate of about 2.0seconds⁻¹ was applied to measure the viscosities below about 900 cP,about 0.8 second⁻¹ to measure the viscosities varying in the range of900 cP to 2,000 cP; and about 0.4 second⁻¹ to measure the viscositiesabove about 2,000 cP.

Mixing Filler and Epoxy to Produce Encapsulant:

The process of mixing the filler particles and the liquid epoxy isschematically illustrated in FIG. 7. This epoxy was reacted at about100° C. for about 120 minutes after addition of the catalyst. Theviscosity of this epoxy, measured at about 100 C was found to be about1,020 cP at a shear rate of about 0.8 second⁻¹. About 30 grams of thisepoxy were combined with about 20 grams of the filler. The resultingliquid suspension had a filler content of about 16 volume percent. Theliquid suspension was mixed in the Keyence mixer for about 2 minutes,followed by cooling down and defoaming in vacuum in an oven at atemperature of about 80° C. for about 20 minutes. The liquid suspensionwas stirred during the steps of cooling and defoaming. In a separateexperiment, a liquid epoxy having the same composition was reacted atabout 100° C. for about 120 minutes in the chamber of the viscometer,and then cooled down from about 100° C. to a temperature of about 50° C.During this cooling, the viscosity of the epoxy was measured in themanner described above. The viscosity of the epoxy (without the filler)was about 8,750 cP at about 80° C. at a shear rate of about 0.4second⁻¹. The resulting hot viscous suspension at about 80° C. was castinto sample molds which had been maintained at a room temperature ofabout 20° C. Finally, the encapsulant was partially post cured at atemperature of about 70° C. for about 2 hours, and then fully post curedat a temperature of about 165° C. for about 4 hours. The encapsulantthereby obtained had dimensions of about 13 mm diameter and about 14 mmlength.

Three samples having a thickness of about 1 mm were cut from thisencapsulant, one each from the top, middle, and bottom. The CTE of thesethree samples was measured using a dynamic mechanical analyzer, ModelNo. DMA 7e, and a 2 millimeter quartz penetration probe, bothmanufactured by Perkin Elmer Analytical Instruments of Shelton, Conn.The sample was heated to a temperature between 30° C. and 190° C. at aheating rate of about 3° C./min. During the measurements, a force ofabout 220 milliNewtons was applied to the probe. The slope of the linearline obtained by best fit to the temperature versus linear thermalexpansion data at the temperature range of 30° C. to 100° C. yielded theCTE value of the encapsulant. The CTE was about 39.2 ppm/° C. in the topsection of the encapsulant, about 38.7 ppm/° C. in the middle section,and about 38.4 ppm/° C. in the bottom section. The average CTE value forthis encapsulant was about 38.8 ppm/° C., with a CTE variation of lessthan about ±1% over its entire volume. This CTE measurement confirmedthat the filler particles were generally uniformly dispersed in theencapsulant. The same dynamic mechanical analyzer measurement was usedto determine that the T_(g) of the encapsulant sample was about 164° C.The transmittance of the fully post cured sample was measured at a roomtemperature of about 20° C. using a visible light spectrometer (Cary 500Scan, marketed by Varian Inc. of Mulgrave, Victoria, Australia). Asample of the encapsulant having a thickness of about 1 millimeter had atransmittance of about 85% when measured at a wavelength of about 650nm, and about 72% when measured at a wavelength of about 450 nm.

COMPARATIVE EXAMPLE 1 Low Viscosity

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that after the addition of TPPP, the solution wasstirred for about 25 minutes at about 100 C, instead of about 120minutes. The viscosity of the resulting epoxy was about 151 Cp at ashear rate of about 2.0 seconds⁻¹ and at about 100° C. Properties of theencapsulant, the filler, and, the epoxy prepared in this comparativeexample, are shown in Table 3. The CTE was about 62.0 ppm/° C. in thetop section of the encapsulant, about 35.0 ppm/° C. in the middlesection, and about 7.5 ppm/° C. in the bottom section. The average CTEvalue for this encapsulant was about 34.8 ppm/° C., with a variation ofabout ±78% over its entire volume. The results of this example indicatedthat the filler particles settled down at the bottom of the epoxy beforeit was solidified. This epoxy had a low viscosity before it was mixedwith the filler, which lead to rapid gravitational settling of theparticles and non-uniform distribution of the filler particles in theencapsulant. The variation of the CTE was therefore unacceptably highfor encasing of opto-electronic devices.

COMPARATIVE EXAMPLE 2 Filler Size Greater than 250 μm

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the particles removed after the step ofball-milling and sieving in EXAMPLE 1 were used in this comparativeexample as a filler. This filler included particles having sizes largerthan about 250 μm. Properties of the encapsulant, the filler, and, theepoxy which were prepared in this example, are shown in Table 3. The CTEwas about 62.0 ppm/° C. in the top section of the encapsulant, about37.0 ppm/° C. in the middle section, and about 15.0 ppm/° C. in thebottom section. The average CTE was about 38.5 ppm/° C., with a CTEvariation of about ±61% over the entire volume of the encapsulant. Thisvariation was caused by gravitational settling of particles larger thanabout 250 μm, which occurred after the filler was mixed with the epoxy.This settling lead to non-uniform dispersion of the filler particles inthe encapsulant and resulted in non-uniform CTE in the encapsulant. Thisencapsulant was not suitable for encasing of opto-electronic devices,because it had non-uniform CTE.

COMPARATIVE EXAMPLE 3 Filler Size Between 1 μm and 100 μm

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the filler particles remaining in the solventused in the step of solvent washing were used as filler. These particleswere recovered by filtration and drying of the solvent. As shown in FIG.8, the size of these particles ranged between 1 μm and 100 μm, with apeak at about 10 μm as determined by the particle size analyzer. Sixtyvolume percent of this powder was composed of particles having sizesless than about 10 μm. Properties of the encapsulant, the filler, and,the epoxy which were prepared in this example, are shown in Table 3. Thetransmittance of the resulting encapsulant was about 48%. The resultsfrom this comparative example showed that the particles smaller thanabout 10 μm were agglomerated in the encapsulant, which lead to decreasein the transmittance.

Taken together, the results of EXAMPLE 1, COMPARATIVE EXAMPLE 2, andCOMPARATIVE EXAMPLE 3 indicated that the preferred size range for thefiller particles is between 1 μm and 500 μm. The more preferred sizerange is between 1 μm and 250 μm in diameter, and the most preferredsize range is between 10 μm and 250 μm. These examples furtherdemonstrated that the filler can contain particles having sizes lessthan 10 μm, with the resulting encapsulant providing acceptable CTE andtransmittance properties. However, the amount of particles smaller thanabout 10 μm in the filler should be preferably lower than about 60volume percent of the filler, more preferably lower than about 20 volumepercent, and most preferably lower than about 10 volume percent.

COMPARATIVE EXAMPLE 4 Encapsulant Incorporating No Filler Particles

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that no filler particles were incorporated into theresulting encapsulant. Properties of the encapsulant, and the epoxywhich were prepared in this example, are shown in Table 3. Thetransmittance of this encapsulant was about 91%. However, the averageCTE was very high, about 62.0 ppm/C.

COMPARATIVE EXAMPLE 5 Lower Filler Amount

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the filler amount in the encapsulant was about 5volume percent, instead of about 16 volume percent. Properties of theencapsulant, the filler, and the epoxy which were prepared in thisexample, are shown in Table 3. The transmittance of this encapsulant wasabout 89.6%. The average CTE was about 55.6 ppm/C with a CTE variationof about ±1%.

EXAMPLE 2 Higher Filler Amount

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the filler amount in the encapsulant was about 46volume percent, instead of about 16 volume percent. Properties of theencapsulant, the filler, and the epoxy which were prepared in thisexample, are shown in Table 3. The transmittance of this encapsulant wasabout 79.2%. The average CTE was about 14.4 ppm/° C. with a CTEvariation less than about ±1%.

Taken together, EXAMPLE 1 and EXAMPLE 2 demonstrated that a wide rangeof filler loading can be incorporated into the encapsulant whileretaining high transmittance, thereby allowing adjustment oftransmittance and CTE to meet varying requirements. Taken together,EXAMPLE 1, EXAMPLE 2, COMPARATIVE EXAMPLE 4, and COMPARATIVE EXAMPLE 5demonstrated that the filler content of the encapsulants should bepreferably between 5 volume percent and 60 volume percent, morepreferably between 10 volume percent and 50 volume percent, and mostpreferably between 15 volume percent and 40 volume percent to obtainboth the low CTE and the high transmittance within the scope of thisinvention.

EXAMPLE 3 No Heat Treatment

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the filler was not heat treated. This filler waskept in atmospheric air at a temperature of about 20° C. for about 1hour before it was combined with the epoxy. Properties of theencapsulant, the filler, and the epoxy which were prepared in thisexample, are shown in Table 3. The transmittance of this encapsulant wasabout 69.0%.

EXAMPLE 4 Heat Treatment at 350° C.

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the filler was heat treated at about 350 C forabout 36 hours, instead of about 500° C. for about 36 hours. Propertiesof the encapsulant, the filler, and the epoxy which were prepared inthis example, are shown in Table 3. The transmittance of thisencapsulant was about 69.0%.

COMPARATIVE EXAMPLE 6 Heat Treatment at 650° C.

An encapsulant was prepared in the same manner as in EXAMPLE 1, exceptthat the filler particles were heat treated at a temperature of about650° C. for about 36 hours, instead of about 500° C. for about 36 hours.This heat treatment temperature was above the strain point of this glassof about 627° C. The resulting encapsulant had reduced transmittance, ofabout 60.5%. This result indicated that the heat treatment temperatureshould be lower than the strain point of the glass.

COMPARATIVE EXAMPLE 7 Heat Treatment at 750° C.

An encapsulant was prepared in the same manner as in EXAMPLE 1, exceptthat the filler particles were heat treated at a temperature of about750° C. for about 36 hours, instead of about 500° C. for about 36 hours.This heat treatment temperature was above the strain point of this glassof about 627° C. and approached the glass softening temperature of about883° C. This heat treatment caused visible aggregation of the fillerparticles. After some mild grinding to separate the filler particles,the encapsulant was produced as described in EXAMPLE 1. The resultingencapsulant had a very low transmittance, of about 46%. This lowertransmittance was caused by formation of air pockets between theaggregated particles.

Taken together, EXAMPLE 1, EXAMPLE 3, and EXAMPLE 4 demonstrated thatthe transmittance of the encapsulant can be maximized by heat treatmentof the filler particles. Taken together, COMPARATIVE EXAMPLE 6 andCOMPARATIVE EXAMPLE 7 demonstrated that the preferred maximum heattreatment temperature for the filler particles used in the encapsulantshould be less than the glass strain point of the filler. The mostpreferred heat treatment temperature range is between 450° C. and 550°C. to maximize the transmittance of the resulting encapsulant.

EXAMPLE 5 Decreased Value of R

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the value of R was about 0.35 instead of about0.40. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. The reduction of R altered therefractive index of the epoxy, resulting in an encapsulant having atransmittance of about 76.5%.

EXAMPLE 6 Increased Value of R

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the value of R was about 0.45 instead of about0.40. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. The increase of R affected therefractive index of the epoxy, resulting in an encapsulant with atransmittance of about 78.1%.

COMPARATIVE EXAMPLE 8 Increased Value of R

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the value of R was about 0.55 instead of about0.40. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. The increase of R affected therefractive index of the epoxy, resulting in an encapsulant having atransmittance of about 64.1%.

COMPARATIVE EXAMPLE 9 Decreased Value of R

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the value of R was about 0.05 instead of about0.40. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. The transmittance of thisencapsulant was very low; specifically, at a value of about 25.0%.

COMPARATIVE EXAMPLE 10 Increased Value of R

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the value of R was about 0.95 instead of about0.40. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. The transmittance of thisencapsulant was very low; specifically, at a value of about 22.0%.

Taken together, EXAMPLE 1, EXAMPLE 5, EXAMPLE 6, COMPARATIVE EXAMPLE 8,COMPARATIVE EXAMPLE 9, and COMPARATIVE EXAMPLE 10, demonstrated that Rshould be preferably between 0.20 and 0.54, and most preferably between0.30 and 0.50, to obtain an encapsulant having a transmittance higherthan 65%.

EXAMPLE 7 Decreased Value of H

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the value of H was about 0.47 instead of about0.63. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. This adjustment affected therefractive index of the epoxy, resulting in an encapsulant havingtransmittance of about 73.8%.

EXAMPLE 8 Increased Value of H

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the value of H was about 0.70 instead of about0.63. Properties of the encapsulant, the filler, and the epoxy preparedin this example are shown in Table 3. This adjustment affected therefractive index of the epoxy, resulting in an encapsulant havingtransmittance of about 82.2%.

COMPARATIVE EXAMPLE 11 Decreased Value of H

An encapsulant was prepared in the same manner as in EXAMPLE 1, exceptthat the amounts of the components of the encapsulant were adjusted sothat the H value was about 0.05 instead of about 0.63. Properties of theencapsulant, the filler, and the epoxy prepared in this example areshown in Table 3. In this Comparative example, the epoxy and the fillermixture did not cure into a solid encapsulant. The result of thisexample demonstrated that solid encapsulants cannot readily be preparedwhen the value of H is lower than the preferred range of the presentinvention.

COMPARATIVE EXAMPLE 12 Increased Value of H

An encapsulant was prepared and analyzed in the same manner as inEXAMPLE 1, except that the amounts of the components of the encapsulantwere adjusted so that the H value was about 0.95 instead of about 0.63.Properties of the encapsulant, the filler, and the epoxy which wereprepared in this example, are shown in Table 3. The transmittance of theresulting encapsulant was very low; specifically, about 21.8%. Theresult of this example showed that the transmittance of the encapsulantwill not be at an optimum value if the H value is higher than thepreferred range of the present invention.

Taken together EXAMPLE 1, EXAMPLE 7, EXAMPLE 8, COMPARATIVE EXAMPLE 11,and COMPARATIVE EXAMPLE 12 demonstrated that the H value should bepreferably between 0.30 and 0.80 and most preferably between 0.50 and0.70 to obtain an encapsulant having a transmittance of higher than 65%.

TABLE 3 Encapsulant Filler Epoxy CTE Trans- Particle Filler Temper-Curing Average Variation mittance Size Amount ature Duration Atmos- RTime Viscosity Examples ppm/° C. ±% % μm Volume % ° C. hours phere 0.40H A minutes cP Example 1 38.8 1 85.0 1-250 16 500 36 Dry air 0.40 0.630.01 120 1020 Comparative 34.8 78 85.0 1-250 16 500 36 Dry air 0.40 0.630.01 25 151 Example 1 Comparative 38.5 61 85.0 ≧250 16 500 36 Dry air0.40 0.63 0.01 120 1020 Example 2 Comparative 38.8 1 48.0 1-90  16 50036 Dry air 0.40 0.63 0.01 120 1020 Example 3 Comparative 62.0 1 91.0 Nofiller — — — 0.40 0.63 0.01 120 1020 Example 4 Comparative 55.1 1 89.61-250 5 500 36 Dry air 0.40 0.63 0.01 120 1020 Example 5 Example 2 14.41 79.2 1-250 46 500 36 Dry air 0.40 0.63 0.01 120 1020 Example 3 38.8 169.0 1-250 16 20 ≧1 — 0.40 0.63 0.01 120 1020 Example 4 38.8 1 69.01-250 16 350 36 Dry air 0.40 0.63 0.01 120 1020 Comparative 38.8 1 60.51-250 16 650 36 Dry air 0.40 0.63 0.01 120 1020 Example 6 Comparative38.8 1 46.0 1-250 16 750 36 Dry air 0.40 0.63 0.01 120 1020 Example 7Example 5 38.8 1 76.5 1-250 16 500 36 Dry air 0.35 0.63 0.01 120 —Example 6 38.8 1 78.1 1-250 16 500 36 Dry air 0.45 0.63 0.01 120 —Comparative 38.8 1 64.1 1-250 16 500 36 Dry air 0.55 0.63 0.01 120 —Example 8 Comparative 38.8 1 25.0 1-250 16 500 36 Dry air 0.05 0.63 0.01120 — Example 9 Comparative 38.8 1 22.0 1-250 16 500 36 Dry air 0.950.63 0.01 120 — Example 10 Example 7 38.8 1 73.8 1-250 16 500 36 Dry air0.40 0.47 0.01 120 — Example 8 38.8 1 82.2 1-250 16 500 36 Dry air 0.400.70 0.01 120 — Comparative Encapsulant did not cure 1-250 16 500 36 Dryair 0.40 0.05 0.01 120 — Example 11 Comparative 38.8 1 21.8 1-250 16 50036 Dry air 0.40 0.95 0.01 120 — Example 12

EXAMPLE 9 Use of Filler Glasses Containing Alkali Ions

In this example, a number of possible glass filler materials, inaddition to the borosilicate glass of the present invention, were used.The glasses used were soda lime glass, marketed under the trade name K5by Schott Corporation, and ZnO—TiO₂ glass, marketed under the productnumber 0211 by Corning Incorporated. The borosilicate glass used inpreferred aspects of the present invention is alkali-free. The alkalicontent of the ZnO—TiO₂ glass is lower than 0.3 weight percent, whilethe alkali content of the soda lime glass is very high, varying between13 weight percent and 17 weight percent. Filler particles were preparedas described in EXAMPLE 1 using each of these three glasses. The epoxycomposition prepared was identical to that described in EXAMPLE 1, andthree encapsulants containing about 40 weight percent of each of thefillers were produced.

The transmittances of these encapsulants were tested immediately afterthey were post-cured, and also after holding them at a temperature ofabout 85° C. in a relative humidity of about 85% for about 100 hours, atwhich point they were tested, and. for about another 1,000 hours, atwhich point they were retested. Specifically, before the step ofpostcuring, three opto-electronic devices were encased with theseencapsulants. After the step of postcuring, the leakage current of eachof these devices was measured to determine the level of electricalresistivity of the encapsulants. The high leakage current suggests thatthe encapsulant had low electrical resistivity, or vice versa. Thesemeasurements were done by attaching the opto-electronic devices to atest fixture (model HP 16442A, manufactured by Hewlett PackardInstruments Inc.), applying approximately 10 volts of potentialdifference to the device, and determining the leakage current using aprecision semiconductor analyzer (model HP 4156A, manufactured byHewlett Packard Instruments Inc.).

The leakage current of the three opto-electronic devices were measuredimmediately after post-curing of the encapsulants, and also afterholding the encased devices at a temperature of about 85° C. in arelative humidity of about 85% for about 100 hours and for about 1,000hours. During this heat and humidity treatment of the encased devices, apotential difference of about 5.25 volts was applied to the devices. Theresults of the transmittance measurements and the leakage current testsare shown in Table 4.

TABLE 4 Alkali-free Property Borosilicate ZnO—TiO₂ Soda Lime AlkaliContent, None Low: <0.3% High: 13-17% weight percent Transmittance (at650 nm): As post-cured 85 85 82 After 100 h 87 80 65 After 1,000 h 87 7825 Leakage Current (pA): As post-cured 23 28 30 After 100 h 23 450 2,300After 1,000 h 23 1,350 70,000

As illustrated by these results, while all three filler materialsproduced encased devices having comparable low leakage current and highencapsulant transmittance at the post-cured stage, only the alkali-freeborosilicate glass maintained these superior properties when exposed tohigh temperature and high humidity for extended time. It appears thatuse in encapsulants of fillers incorporating alkali ions such as sodiumor potassium, leads to deterioration of the electrically insulatingproperties, as well as the transmittance, of the encapsulants afterextended exposure to high humidity and high temperature environments.

This example shows that the alkali-free and the titania-free glasses arethe most preferred glasses for use in encapsulants for opto-electronicdevices within the scope of this invention, since they both initiallyprovide and subsequently maintain high electrical resistivity as well ashigh transmittance. Other titania-free glasses, such as soda-limeglasses, also can be used as fillers of encapsulants for electronicdevices that can tolerate low electrical resistivity, or as fillers ofepoxies for non-electronic applications not requiring electricalinsulation such as lenses.

Although the invention has been described in detail with reference onlyto the presently preferred encapsulants and methods of preparation,those of ordinary skill in the art will appreciate that variousmodifications can be made without departing from the invention.Accordingly, the invention is defined only by the following claims.

1. A method for making an encapsulant for an opto-electronic device oroptical component, comprising: processing a glass characterized by aglass refractive index having a value in the range of 1.48 to 1.60 witha variance of less than about 0.001 to form filler particles from theglass having diameters between 1 μm and 500 μm and characterized by afiller refractive index; heating the filler particles in anoxygen-containing atmosphere; preparing an epoxy including at least acuring agent and characterized by an epoxy refractive index, wherein thefiller refractive index and the epoxy refractive index are sufficientlysimilar such that the encapsulant has optical transmittance of at least65% when measured at a wavelength in the range of 300 nm to 800 nm at anencapsulant thickness of about 1 mm; heating the epoxy to apredetermined temperature for a predetermined duration sufficient toincrease the viscosity of the epoxy to a level such that the epoxy ischaracterized by a settling velocity equal to or greater than apredetermined value; mixing the epoxy with the filler particles within apredetermined mixing duration to form the encapsulant; cooling theencapsulant to a predetermined temperature sufficient to increase theviscosity of the epoxy in the encapsulant within a predetermined coolingduration; and removing air bubbles from the encapsulant within apredetermined defoaming duration.
 2. A method as defined in claim 1,wherein the step of processing a glass comprises processing the glass toform filler particles having diameters between 1 μm and 250 μm.
 3. Amethod as defined in claim 2, wherein the step of processing a glasscomprises processing the glass to form filler particles, such that lessthan 60 percent by volume of the filler particles have diameters lessthan 10 μm.
 4. A method as defined in claim 3, wherein the step ofprocessing a glass comprises processing the glass to form fillerparticles, such that less than 20 percent by volume of the fillerparticles have diameters less than 10 μm.
 5. A method as defined inclaim 4, wherein the step of processing a glass comprises processing theglass to form filler particles, such that less than 10 percent by volumeof the filler particles have diameters less than 10 μm.
 6. A method asdefined in claim 5, wherein the step of processing a glass comprisesprocessing the glass to form filler particles having diameters between10 μm and 250 μm.
 7. A method as defined in claim 1, wherein the step ofheating the filler particles comprises heating the filler particles to atemperature about the strain point of the glass or less.
 8. A method asdefined in claim 7, wherein the step of heating the filler particlescomprises heating the filler particles to a temperature between 20° C.and the strain point of the glass.
 9. A method as defined in claim 7,wherein the step of heating the filler particles comprises heating thefiller particles to a temperature between 20° C. and 627° C.
 10. Amethod as defined in claim 9, wherein the step of heating the fillerparticles comprises heating the filler particles to a temperaturebetween 450° C. and 550° C.
 11. A method as defined in claim 7, whereinthe step of heating the filler particles comprises heating the fillerparticles for a duration of at least about 1 hour.
 12. A method asdefined in claim 11, wherein the step of heating the filler particlescomprises heating the filler particles for a duration of between 5 hoursand 50 hours.
 13. A method as defined in claim 12, wherein the step ofheating the filler particles comprises heating the filler particles fora duration of between 30 hours and 40 hours.
 14. A method as defined inclaim 1, wherein the step of heating the filler particles comprisesheating the filler particles in dry air.
 15. A method as defined inclaim 1, further comprising a step of reacting the glass particles witha silane coupling agent after the step of processing the glass.
 16. Amethod as defined in claim 15, wherein the silane coupling agent isaminopropyltriethoxysilane.
 17. A method as defined in claim 1, whereinthe step of preparing an epoxy comprises mixing together a diglycidylether of bisphenol-A resin, a cyclo-aliphatic resin, a hardener, and anaccelerator.
 18. A method as defined in claim 1, wherein the step ofpreparing an epoxy comprises preparing the epoxy to have a sufficientviscosity such that the settling velocity of the filler particles in theencapsulant is less than about 11 mm/min.
 19. A method as defined inclaim 18, wherein the step of preparing the epoxy to have a sufficientviscosity comprises preparing the epoxy such that the settling velocityof the filler particles in the encapsulant is less than about 6 mm/min.20. A method as defined in claim 19, wherein the step of preparing theepoxy to have a sufficient viscosity comprises preparing the epoxy suchthat the settling velocity of the filler particles in the encapsulant isless than about 4 mm/min.
 21. A method as defined in claim 1, furthercomprising heating the epoxy to a temperature between 80° C. and 140° C.for a predetermined duration sufficient to increase the viscosity of theepoxy before the step of mixing the epoxy with the filler particles. 22.A method as defined in claim 21, wherein the step of heating the epoxycomprises heating the epoxy to a temperature between 90° C. and 110° C.23. A method as defined in claim 22, wherein the step of heating theepoxy comprises heating the epoxy to a temperature of about 100° C. 24.A method as defined in claim 23, wherein the step of heating the epoxycomprises heating the epoxy for a duration sufficient to increase theviscosity of the epoxy to be in the range of 300 cP to 40,000 cP.
 25. Amethod as defined in claim 24, wherein the step of heating the epoxycomprises heating the epoxy for a duration sufficient to increase theviscosity of the epoxy to be in the range of 500 cP to 20,000 cP.
 26. Amethod as defined in claim 25, wherein the step of heating the epoxycomprises heating the epoxy for a duration sufficient to increase theviscosity of the epoxy to be in the range of 750 cP to 10,000 cP.
 27. Amethod as defined in claim 1, wherein the step of cooling theencapsulant comprises cooling the encapsulant to a temperaturesufficient to increase the viscosity of the epoxy to a value in therange of 1,000 cP to 40,000 cP.
 28. A method as defined in claim 27,wherein the step of cooling the encapsulant comprises cooling theencapsulant to a temperature sufficient to increase the viscosity of theepoxy to a value in the range of 5,000 cP to 20,000 cP.
 29. A method asdefined in claim 28, wherein the step of cooling the encapsulantcomprises cooling the encapsulant to a temperature sufficient toincrease the viscosity of the epoxy to a value in the range of 7,000 cPto 12,000 cP.
 30. A method as defined in claim 1, wherein the steps ofmixing, cooling, and removing air bubbles have a combined duration ofless than about 120 minutes.
 31. A method as defined in claim 30,wherein the steps of mixing, cooling, and removing air bubbles have acombined duration of less than about 60 minutes.
 32. A method as definedin claim 31, wherein the steps of mixing, cooling, and removing airbubbles have a combined duration of less than about 30 minutes.
 33. Amethod for making an encapsulant for an opto-electronic device oroptical component, comprising: processing a borosilicate glasscharacterized by a refractive index having a value of about 1.526 whenmeasured at a wavelength of about 588 nm and a variance of less thanabout 0.001, to form borosilicate glass particles having diametersbetween 1 μm and 250 μm, such that less than 60 percent by volume of theborosilicate glass particles have diameters less than 10 μm; heating theborosilicate glass particles to a temperature between 450° C. and 550°C. for a duration between 30 hours and 40 hours in an oxygen-containingatmosphere; reacting the borosilicate glass particles withaminopropyltriethoxysilane; preparing an epoxy by mixing: a) diglycidylether of bisphenol-A resin; b) cyclo-aliphatic resin; c)hexahydrophthalic anhydride curing agent; and d) triphenyiphosphitecatalyst; heating the epoxy to about 100° C. for a duration sufficientto increase the viscosity of the epoxy to between 750 cP and 10,000 cPto obtain a settling velocity of the borosilicate glass particles in theepoxy of less than 4 mm/min; mixing the epoxy and the borosilicate glassparticles; cooling the encapsulant to a temperature sufficient tofurther increase the viscosity of the liquid epoxy to a level between7,000 cP and 12,000 cP; and removing air bubbles from the encapsulant;such that the steps of mixing, cooling, and removing have a combinedduration of less than about 30 minutes.